WINDSOR, Ontario -- Trucks carrying nearly a quarter of the $1.3 billion in daily trade between the U.S. and Canada rumble single file across the Ambassador Bridge here, as if through a funnel, before passing through U.S. customs booths just off the Detroit end of the span.

The line flows pretty smoothly these days -- a far cry from the 18-mile backup that followed the Sept. 11 terrorist attacks on the U.S., when security was heightened and before economic sluggishness cut down on the traffic.